Protection against indirect contact
By automatic disconnection of the circuit – The Standard IEC 60364 prescribes automatic disconnection of the supply for protection against indirect contact.
The protective device shall automatically disconnect the supply so that, in the event of a fault between a live part and an exposed-conductive-part or a protective conductor, a prospective touch voltage exceeding 50 V a.c. (25 V in special environments) does not persist for a time sufficient to cause a risk of harmful physiological effect in a person in contact with simultaneously acces- sible conductive parts.
This protective measure requires co-ordination between the connection to earth of the system and the characteristics of the protective conductors and devices.
The devices suitable for the automatic disconnection of the supply and able to detect earth fault currents are:
- Automatic circuit-breakers with thermomagnetic release;
- Automatic circuit-breakers with microprocessor-based electronic relay;
- Automatic circuit-breakers with microprocessor-based electronic relay with integrated protection against earth fault (function G);
- Thermal magnetic or electronic circuit-breakers with integrated residual current releases;
- Pure residual current circuit-breakers;
- Residual current releases.
Hereunder there is a description of such protective devices.
Automatic circuit-breakers with thermomagnetic release //
The protections ensured by the automatic circuit-break- ers equipped with thermomagnetic release are:
- Protection against overloads;
- Protection against short-circuits;
- Protection against indirect contacts.
The protection against overload is provided by the thermal release with inverse time-delay curve, i.e. the higher the overload current, the faster the tripping time.
The protection against short-circuit is provided through the magnetic release with an indipendent time trip curve, i.e with disconnecting time independent from the short- circuit current.
The protection against indirect contacts can be carried out both by the thermal release as well as by the magnetic release since the earth fault current involves at least one phase. If this current is high enough, it can cause the tripping of the circuit-breaker.
As explained farther in this paper, it is necessary that the protective device is coordinated with the distribution system and the earthing modality of the exposed conductive-parts, so that tripping is guaranteed to occur in such times to limit the persistence of the dangerous touch voltages present in the exposed-conductive-parts further to the fault.
Figure above shows an example of the earth fault current path in a system with the neutral is directly earthed and the exposed-conductive-parts are connected to the same earthing arrangement of the neutral (TN system) and the trip curve of a thermal magnetic circuit-breaker type Tmax T1C160 R160.
|Title:||Distribution systems and protection against indirect contact and earth fault – ABB|
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